87 TBI 305 runs wide open throttle

[dont77]

Need some help with subject truck when started runs wideopen throttle.  When parked a couple of years ago, and before I got it, the truck had fuel pump and tank switch problems.  Switch has been replaced and fuel pump replaced in right tank.  Left tank needs new tank, etc.  I will replace as I restore the truck.  I just want to get the truck running to determine engine and transmission status to see if any major issues.  I added fuel stabilizer and did injector cleaner to TBI before attempting to start.  It started instantly but runs WOT.  I only let run for seconds, tried again with same result and shut down immediately.  I'm not up to speed on electronics, switches, relays in the transformation away from carbs, etc.  Anyone out there know where I should start?  Thanks in advance!  DT

[bd]

Is the Idle Air Control (IAC) unplugged from the right side of the throttle body (the square, 4-wire, black connector)?  Do you know how to retrieve trouble codes?  If not, refer to this link:  Retrieving Trouble Codes.  Record any stored trouble codes before continuing below.

It sounds like the IAC is hanging open much too far.  While you're looking at basics, check the two 10-amp ECM fuses in the fuse block - one is ignition, the other is battery power

[dont77]

Thanks for the come back!  IAC is connected, but I am not sure if serviceable.  I plan to get buddies  "Snap On Computer" and download Codes.  I was told that once the battery is disconnected the computer (ECM) looses all codes.  However, buddy tells me there should be "stored codes" still available.  I've also been told that simply turning the ignition key to on, without starting the engine, will not give codes necessary to start trouble shooting?  Even just the electronic issues.  I'm not sure due to lack of experience.  How long does the truck have to run to get codes to display?  I don't want to run a cold, much less a warm engine, at wide open throttle.  I will keep you posted on codes and progress.  Again, many thanks.  DT

[bd]

Thanks for the come back!  IAC is connected, but I am not sure if serviceable.  I plan to get buddies  "Snap On Computer" and download Codes.  I was told that once the battery is disconnected the computer (ECM) looses all codes.  However, buddy tells me there should be "stored codes" still available.  I've also been told that simply turning the ignition key to on, without starting the engine, will not give codes necessary to start trouble shooting?  Even just the electronic issues.  I'm not sure due to lack of experience.  How long does the truck have to run to get codes to display?  I don't want to run a cold, much less a warm engine, at wide open throttle.  I will keep you posted on codes and progress.  Again, many thanks.  DT

Disconnecting the battery will erase ALL codes.  None will remain stored in memory!  FYI - Disconnecting the battery will also erase the adaptive learn memory, which will not cause a problem for you, but can slightly alter the running of the vehicle until it relearns your driving habits.  Adaptive learn can be ignored, since it is not important to your diagnosis.

Retrieving stored trouble codes from memory does not require the engine to be running.  And, retrieval doesn't erase the code(s).  Stored codes are just that, stored codes.  They provide a starting point for troubleshooting.  However, the "live data" used to augment troubleshooting can only be monitored/retrieved while the engine is running. 

Depending on the existing problem, merely switching the ignition ON can register certain codes.  While many codes require the engine to run long enough for a fault to occur, before setting.  Intermittent faults can require a significant amount of windshield time to set a code and diagnose.

Be aware that while the ignition is ON, unplugging sensors and actuators may set "false" trouble codes that can confuse your diagnosis.

-------------------

First suspicion is faulty IAC control signals or faulty IAC.  Another possibility is that the IAC pintle has been jammed by something falling into the air passage.

[dont77]

Thanks Rich for the information!  If I may, just so I can try and understand how the air flow, and not an excessive fuel flow, could be causing the engine to run wide open?  My first thought was that the fuel pump to be "over pumping" or "over pressuring" the flow of fuel into the TBI, thus causing it to make it run wide open (just like a stuck throttle could do on a carb'd engine).  My thought was (right, wrong, or indifferent) that the TBI wass being fed too much fuel.  I can't get into my mind - how too much air could be doing this?  Then again, as I stated, I am old school.  Please bear with me!  I plan to get diagnostic machine and see what codes may still be resident and try getting codes to appear with just the switch on.  I have read that IAC can be cleaned but to not to completely disassemble (pintle specifically).  May be best to just buy a new IAC and go from there.  This is nothing like my (now seems simple) restoration of a 66 Mustang.  I do appreciate your information and patience.  Thanks again. DT

[bd]

Thanks Rich for the information!  If I may, just so I can try and understand how the air flow, and not an excessive fuel flow, could be causing the engine to run wide open?  My first thought was that the fuel pump to be "over pumping" or "over pressuring" the flow of fuel into the TBI, thus causing it to make it run wide open (just like a stuck throttle could do on a carb'd engine).  My thought was (right, wrong, or indifferent) that the TBI was being fed too much fuel.  I can't get into my mind - how too much air could be doing this?  Then again, as I stated, I am old school.  Please bear with me!  I plan to get diagnostic machine and see what codes may still be resident and try getting codes to appear with just the switch on.  I have read that IAC can be cleaned but to not to completely disassemble (pintle specifically).  May be best to just buy a new IAC and go from there.  This is nothing like my (now seems simple) restoration of a 66 Mustang.  I do appreciate your information and patience.  Thanks again. DT

Actually it's "airflow" into and through an engine that controls RPM, whether the air is fed to the engine via carburetion or injection makes no difference.  When you open the throttle on a carburetor or TBI, etc, you're mechanically opening a butterfly valve, allowing more air into the engine. 

Fundamentally, a carburetor "mechanically" matches fuel delivery to airflow, based on analog pressure differentials.  Whereas, TBI or other electronic injection, adjusts fuel delivery to match airflow via analog sensors that are digitally interpreted by an "electronic" microprocessor, which in response adjusts fuel injector pulse width (injector ON time).  The longer the injector ON time the greater the fuel delivery.  The microprocessor (ECM) also may control other engine and powertrain functions, such as timing, idle speed, transmission shift points, air-conditioning, etc.

In TBI systems, the throttle plates are nearly closed at idle, more completely closed than carburetors are, yet open just enough to prevent the plates from seating in the throttle bores and sticking closed.  Idle speed is adjusted by the position of the IAC pintle valve, which is just an ECM controlled throttle bypass.  As the pintle moves fore and aft in its bore, it allows less or more air to bypass the throttle plates.  More air into the engine equates to higher idle speed.  The IAC serves other functions, as well.

To maintain programmed RPM, the ECM monitors tach signal and continuously adjusts the IAC.  Fuel delivery is electronically adjusted to compensate for idle airflow via continuous feedback to the ECM from the Manifold Absolute Pressure (MAP) or Vacuum Sensor, the Throttle Position Sensor (TPS), the Coolant Temperature Sensor (CTS), and engine RPM. 

If you're familiar with carburetion, there are analogies between the various circuits of a carburetor and the sensors used with TBI.  To help you become more comfortable and better think with TBI, conceptualize the following:  the CTS is analogous to a carburetor choke circuit adjusting fuel delivery according to engine temperature, the TPS is analogous to an accelerator pump circuit wherein the rate of throttle opening modifies fuel delivery, the MAP is analogous to a power enrichment circuit by monitoring engine load, fuel pressure is loosely analogous to float level, the fuel injectors are analogous to the main nozzles, and the IAC is analogous to an idle solenoid controlling idle RPM.  Of course it isn't quite that simple and some functions overlap, but you get the idea, nonetheless.  Fuel delivery and other functions are then controlled by the ECM according to a fixed program table or "map."  The more you work with TBI, the simpler it will become.

[dont77]

Rich - very good information.  Give me a couple of days to apply what you have provided and I will let you know how it turns out.  I purchased a new IAC today.  I plan to install and see if that solves the WOT.  If not, I will try and download what codes I can and go from there.  I'm hoping I can get WOT to cease as to allow the engine to run normal to get applicable codes.  I apprecite all you expertise.  Do have a good one.  DT

[Irish_Alley]

vacuum leak

[mdws1987]

This may be over too simple but, does the truck have cruise control? I had this problem and it was the cruise module pushing the TBI to WOT. I removed the linkage from the TBI and got normal idle..

[dont77]

Yes, the truck does have cruise control.  Can you be specific as to what you disconnected so I can give this a try?  Unless the wire, or other apparatus being disconnected has a bearing on the starting or running of the engine, this is definitely worth the time and effort!   Many thanks for the heads-up.  DT

[74 C-10 Shorty]

I believe he said the linkage off the TBI, could have someone watch what's going on when you start it up???..

[bd]

This may be over too simple but, does the truck have cruise control? I had this problem and it was the cruise module pushing the TBI to WOT. I removed the linkage from the TBI and got normal idle..

Yes, the truck does have cruise control.  Can you be specific as to what you disconnected so I can give this a try?  Unless the wire, or other apparatus being disconnected has a bearing on the starting or running of the engine, this is definitely worth the time and effort!   Many thanks for the heads-up.  DT

The simplest solutions are often the best and easy to overlook.  There's an adjustable, solid steel rod that links the cruise control vacuum module to the TBI throttle.  Sometimes the link will bind, because of wear or misalignment, where it hooks onto the vacuum actuator and hold the throttle open.  Remove the retaining clip and slide the rod from the throttle crank.  If that's the culprit, you'll know instantly.

[dont77]

Thanks again to all.  It will be the weekend before regular job will allow me to apply what has been passed herein.  Looking forward to finding the culprit and learning along the way.  DT

[dont77]

LOW AND BEHOLD, THE PROBLEM WAS WITH A STICKING CRUISE CONTROL LINKAGE.  THE SAID LINKAGE WAS HUNG UP ABOUT .5 INCH, CAUSING THE WOT.  ONCE RELEASED AND TURN OF THE KEY, THE ENGINE STARTED AND SMOOTHED OUT QUICKLY TO A STEADY IDLE.  NO CHECK ENGINE LIGHT.  TEST DRIVE PROVED UNEVENTFUL.  PLESESANTLY SURPRISED SINCE TRUCK HAS SET UNTOCUHED FOR TWO YEARS.  MANY THANKS TO ALL THAT PROVIDED VERY USEFUL TROUBLESHOOTING INFORMATION.  SINCE I HAD A NEW IAC I REPLACED IT ALSO.  LOOKING FORWARD TO MOVING FORWARD ON TOTAL RESTORATION/MODIFICATION AND i AM SURE I WILL BE POSTING MORE "HELP MES"!  THANKS GENTS!  DT